utf8.c: Add _flags version of to_utf8_fold()

utf8.c: Add _flags version of to_utf8_fold()

And also to_uni_fold().

The flag allows retrieving either simple or full folds.

The interface is subject to change, so these are marked experimental
and their names begin with underscore.  The old versions are turned
into macros calling the new versions with the correct extra parameter.